Nigeria's main militant group has released pictures of two British oil workers it has been holding hostage for four months, saying it would not relent on kidnapping foreign oil workers in the campaign for a greater share of the region's oil resources.

The two hostages were part of a group of more than 20 oil workers taken hostage by gunmen who hijacked their oil supply vessel in the restive Niger Delta. The Movement for the Emancipation of the Niger Delta, MEND, said it rescued the hostages from the original captors and released all but the two British nationals.

MEND says the hostages will remain captive until its leader, Henry Okah, who is facing treason and gun-running charges, is freed by Nigeria. They also want Britain to end its involvement with training Nigerian troops.

Kidnappings are commonplace in Nigeria's southern oil region, with hundreds of people seized in the past three years.

A former militant leader, Mujaheed Dokubo-Asari, says kidnappings and stealing oil is giving the Niger Delta struggle a bad name.

"The abduction, the kidnapping, the stealing - it is not part of our struggle. If anybody accepts it and say it has helped our struggle, I will say 'no' it has not helped our struggle in any way. It has made us to lose sympathy from people who sympathize with us," he said.

A spokeswoman for the British High Commission in Abuja has urged the rebels to release the hostages immediately and without conditions.

Attacks by MEND and other armed groups on Nigeria's oil industry have cut the country's exports by more than 20 percent since 2006. The country is Africa's leading oil exporter and the fifth biggest source of U.S. oil imports.